Device to prevent overpressure in a capacitor or ultracapacitor

ABSTRACT

The present invention relates to a device to prevent overpressure in a supercapacitor. In a supercapacitor comprising a closed chamber fitted with means for exchanging a gas with the external surroundings and in which there are positioned two electrodes with a high specific surface area, separated by a separator, the separator and the electrodes being impregnated with an electrolyte, the means for exchanging a gas comprise a metallic membrane that is permeable to hydrogen and its isotopes and impermeable to gaseous species which have an effective cross section of 0.3 nm or higher, at a temperature of between −50° C. and 100° C.

RELATED APPLICATIONS

This application is a National Phase application of PCT/FR2009/000161,filed on Feb. 13, 2009, which in turn claims the benefit of priorityfrom French Patent Application No. 08 00813, filed on Feb. 14, 2008, theentirety of which are incorporated herein by reference

BACKGROUND

1. Field of the Invention

The present invention relates to supercapacitors, and more particularlyto a device to prevent overpressure for a supercapacitor.

2. Description of Related Art

Various electrochemical devices, in particular supercapacitors, producehydrogen during their operation.

A supercapacitor comprises two electrodes with a high specific surfacearea, between which a separator is placed, this assembly being placed ina closed chamber. The separator and the electrodes are impregnated witha solution of an ionic compound in a liquid solvent.

The supercapacitor generates gas during operation, which is essentiallyhydrogen. Buildup of the hydrogen formed in the supercapacitor causes anincrease in the internal pressure, which is detrimental to the lifetimeof the supercapacitor. An internal overpressure can degrade thesupercapacitor by deformation, by opening or by explosion.

Various devices have been proposed in the prior art in order to remedythis problem.

Reversible degassing valves are used particularly in lead batteries,referred to as VRLA. They consist of a polymer membrane, in particular apolyethylene membrane. These membranes are not suitable forsupercapacitors because they do not prevent entry of air into thedevice.

Various supercapacitors, in particular some marketed by the companiesMaxwell or Epcos, are designed so that the casing has a weak zone whichruptures when the internal pressure exceeds a given threshold. Althoughsuch a device avoids any catastrophic behavior of the capacitor (inparticular due to explosion), it nevertheless has the drawback of beingirreversible and consequently does not allow the lifetime of thesupercapacitor to be increased.

Reversible degassing valves exist on various supercapacitors marketed bythe company Nippon-Chemicon. In these supercapacitors, the degassingvalve comprises an elastomer seal held under pressure by a washer. Theliquid of the electrolyte is propylene carbonate (PC) which is a liquidwith low volatility, so as to prevent or at least limit deposition ofthe salt of the electrolyte in the valve. However, when the electrolyteis a salt in solution in a volatile solvent, for example acetonitrile,the risk of the valve being clogged by the salt increases significantly.In fact, a salt deposit at a valve will irreversibly lead to entry ofair and water into the supercapacitor. It is well known that water andoxygen are highly reactive chemical species which rapidly degrade theproperties of the electrolyte (and potentially the electrodes) therebyvery rapidly leading to the end of life of the supercapacitor (U.S. Pat.No. 6,233,135).

The use of acetonitrile compared with propylene carbonate in asupercapacitor is desirable because an electrolyte in which the solventis acetonitrile has a conductivity higher than that of an electrolyte inwhich the liquid solvent is PC. Furthermore, the generation of gas isgreater over the course of time in the supercapacitor when the solventis PC. However, the internal overpressure of a supercapacitor leads toits end of life by deformation, by opening or by explosion. For the sameageing conditions, a supercapacitor operating with an electrolyte basedon PC therefore generally exhibits a shorter lifetime than when theelectrolyte is based on acetonitrile.

DE-10 2005 033 476 describes a device which uses a polymer membrane withselective permeability. The membrane is a so-called “non-porous”membrane through which a gas can pass by diffusion, and not by directpassage. It is in particular a polymer membrane, in particular an EPDMmembrane. The elasticity of such a polymer membrane makes it possible toattenuate strong productions of gas inside the device because themembrane can form a bubble, which increases the surface area fortransfer to the outside, for example when an increase in temperaturecauses an increase in the production rate of the gas. However, polymermembranes do not prevent reverse diffusion of undesirable gases such asoxygen, water vapor, carbon monoxide and dioxide, nitrogen oxides or anyother gas which is sufficiently small but detrimental to the ageing ofsupercapacitors which operate in an organic medium or in an aqueousmedium.

Numerous metals exhibit permeability to hydrogen. When a membraneconsisting of such a metal is placed in a gas flow containing hydrogen,the hydrogen gas dissociates in contact with the membrane's face exposedto the gas flow, the dissociated hydrogen diffuses through the membraneand recombines when it reaches the opposite face of the membrane, andmolecular hydrogen escapes from the membrane.

Information relating to the selective permeability of various metals andmetal alloys in relation to hydrogen and its isotopes can be found inthe literature. In particular, mention may be made of “Review ofHydrogen Isotope Permeability Through Materials”, by S. A. Steward,Lawrence Livermore National Laboratory, University of California, 15Aug. 1983, which gives data associated with metals and metal alloys, inparticular those in the table below.

Φ_(25° C.) Φ_(70° C.) Φ₀ (mol · m⁻¹ · Metal (mol · m⁻¹ · s⁻¹ ·Pa^(−1/2)) E_(Φ) (K) s⁻¹ · Pa^(−1/2)) Aluminum^(†)   3 10⁻⁵ 14800 8.110⁻²⁷ 5.5 10⁻²⁴ Copper 8.4 10⁻⁷ 9290 2.4 10⁻²⁰ 1.4 10⁻¹⁸ Stainless   110⁻⁷ 8000 2.2 10⁻¹⁹ 7.4 10⁻¹⁸ Steel Nickel 3.9 10⁻⁷ 6600 9.4 10⁻¹⁷ 1.710⁻¹⁵ Palladium 2.2 10⁻⁷ 1885 3.9 10⁻¹⁰ 9.0 10⁻¹⁰ ^(†)Average value,depending on the surface quality; E_(Φ max) = 18900 K.

U.S. Pat. No. 3,350,846 describes a method of recovering hydrogen bypermeation through metallic membranes which allow selective diffusion ofH₂. The membranes consist of Pd, a PdAg alloy, or alternatively theycomprise a layer of a group VB metal (V, Ta, Nb) coated on each of itsfaces with a continuous non-porous film of Pd or an alloy of PdAg, PdAuor PdB. In a preferred embodiment, the membranes are heated to atemperature of between 300° C. and 700° C., a temperature range which isincompatible with an application of the supercapacitor type.

The site http://www.ceth.fr/sepmemfr.php describes a method of purifyinga gas using a metallic membrane allowing hydrogen to be separatedselectively from a gas mixture. The membrane is an all-metal compositemembrane consisting of three layers. A very fine but dense layer ofpalladium constitutes the active part providing the selectivepermeability. It is supported by a thin metallic intermediate layer withfine pores, which makes it possible to ensure a very good holding of thedense palladium layer even at high levels of temperature or pressure.The intermediate layer is itself supported by a thicker porous metallicsubstrate. The hydrogen molecules which arrive in contact with thepalladium layer are adsorbed and dissociated, and the elements resultingfrom the disassociation diffuse through the palladium layer andrecombine when they desorb from the palladium.

U.S. Pat. No. 4,468,235 describes a method for extracting H₂ containedin a mixture of fluids by bringing the mixture of fluids (liquid orgaseous) in contact with a membrane consisting of a titanium alloycontaining ˜13% V, ˜11% Cr and ˜3% Al and carrying a metal selected fromamong Pd, Ni, Co, Fe, V, Nb or Ta, or an alloy containing one of thesemetals, on one of its faces.

Pd alloys, such as for example PdAg, PdCu, PdY, are considered to havegood mechanical endurance to hydrogen and a permeability higher thanthat of palladium on its own (in particular Pd₇₅Ag₂₅). For example, U.S.Pat. No. 2,773,561 gives a comparison of the hydrogen permeability[expressed in cm³/s/cm²] of Pd and an alloy Pd₇₅Ag₂₅, which issummarized in the following table for membranes having a thickness of25.4 μm.

450° C. 550° C. Pressure (MPa) Pd PdAg Pd PdAg 0.69 0.71 1.22 1.08 1.411.38 1.23 1.93 1.86 2.32 2.07 1.68 2.56 2.42 2.99

It is furthermore known that for an alloy Pd_(100-x)Cu_(x) in whichx<30, the diffusion coefficient remains unchanged but the activationenergy of the diffusion is about ⅓ that of Pd, and that the permeabilityΦ consequently increases, according to the equation

${\Phi = {\Phi_{0}{\mathbb{e}}^{- \frac{E_{\Phi}}{T}}}},$in which Φ₀ is a constant (in mol.m⁻¹.s⁻¹.Pa^(−1/2)), E_(Φ) (in kelvin)is the activation energy of the diffusion, and T is the temperature (inK) (cf. “Diffusion of hydrogen in copper-palladium alloys”, J. Piper, J.Appl. Phys. Vol. 37, 715-721, 1966).

The hydrogen permeability of membranes consisting of Pd or Ni isdescribed in particular in “Hydrogen permeability measurement throughPd, Ni and Fe membranes,” K. Yamakawa et al., J. Alloys and Compounds321, 17-23, 2001.

Alloys based on palladium-silver are considered to exhibit efficientdiffusion for hydrogen, in particular in “Investigation ofElectromigration and Diffusion of Hydrogen in Palladium and PdAg Alloy”,R. Pietrzak et al., Defect and Diffusion Forum, vol 143-147, 951-956,1997).

Membranes consisting of alloys of Pd (PdAg, PdY) on a ceramic supportare selective for the separation of hydrogen from a gas mixture. [Cf.“Catalytic membrane reactors for tritium recovery from tritiated waterin the ITER fuel cycle”, S. Tosti et al., Fusion Engineering and Design,Vol. 49-50, 953-958, 2000)].

U.S. Pat. No. 6,800,392 also describes the use of a membrane consistingof an alloy of Nb with from 5 to 25% of another metal selected fromamong Pd, Ru, Rh, Pt Au and Rh, the alloy membrane being obtained bycolaminating films with different constituents. It is mentioned that thesolubility of hydrogen in an alloy NbPd is about two times that of analloy PdAg₂₃.

Niobium has a very high permeability and is considered as the materialmost permeable to hydrogen in the study by REB Research & Consultingavailable at http://www.rebresearch.com/H2perm2.htm, from which FIG. 1representing the permeability P in mol/mPa^(1/2)s as a function of 1/T(K⁻¹) is taken.

A permeability value of 3.2 10⁻⁷ mol.m⁻¹.s⁻¹.Pa^(−1/2) at 425° C. isfurthermore put forward in Journal of Membrane Science, Vol. 85, 29-38,1993. These properties, however, do not seem to be as beneficial at thetemperatures at which supercapacitors operate (<100° C.). In particular,hydrogen forms a compound with niobium which is stable at lowtemperature, which mechanically weakens the niobium and limits thediffusion of hydrogen (cf. “Extractive Metallurgy of Niobium”, C. K.Gupta, CRC Press, 1994). Furthermore, niobium oxidizes very easily atroom temperature. A barrier layer against the entry of hydrogen into thematerial is then formed on the surface. At room temperature, it is thephenomenon of an adsorption which most limits the diffusion of hydrogenthrough a niobium membrane. This is why the majority of authorspublishing work on niobium report having worked with niobium coveredwith a very thin layer of palladium (thickness <1 μm): the palladiumavoids the surface oxidation problems (its oxide is immediately reducedin the presence of hydrogen) and promotes the adsorption of hydrogen.

These reservations also apply to tantalum and vanadium. Although thesematerials seem beneficial at high temperature (>400° C.), at lowertemperatures they have the same deficiencies as niobium: oxidationlayer, weakening linked with the formation of stable metal-H_(x)compounds, low adsorption power. Here again, specialists generallyrecommend depositing a thin layer of palladium on the surface of thematerial for correct operation.

V—Ti—Ni alloys have a high hydrogen permeability, in particular thealloy V₅₃Ti₂₆Ni₂₁ whose permeability is 1.0-3.7 10⁻⁹mol.m⁻¹.s⁻¹.Pa^(−1/2): at 22° C., which is a value higher than that ofpalladium, namely 3.3-4.3 10⁻¹⁰. (Cf. “Hydrogen Permeability ofMultiphase V—Ti—Ni Metallic Membranes”, Report under Contract No.DE-AC09-96SR18500 with the U.S. Department of Energy, T. M. Adams, J.Mickalonis).

OBJECTS AND SUMMARY

Membranes of V-15% Ni-0.05% Ti or V-15% Ni-0.05% Y with a thin depositof palladium have highly beneficial permeability values (6 10⁻⁸mol.m⁻¹.s⁻¹.Pa^(−1/2) at 200° C.). However, a decrease of this valuewith time is observed (−30% after one week) which could limit theirbenefit for long-term applications such as the one envisaged in thepresent document. (Cf. “V—Ni Alloy Membranes for Hydrogen Purification”,Nishimura et al., JAC 330-332 (2002), pp 902-906).

It is an object of the present invention to provide a device whichallows the hydrogen formed inside a supercapacitor to be removed rapidlyand selectively, while preventing the passage of any other gas from theoutside to the interior of the supercapacitor.

BRIEF DESCRIPTION OF THE DRAWINGS:

The present invention can be best understood through the followingdescription and accompanying drawings, wherein:

FIG. 1 represents the permeability P of Niobium in mol/mPa^(1/2)s as afunction of 1/T (K⁻¹);

FIG. 2 illustrates a membrane of a supercapacitor according to oneembodiment;

FIG. 3 shows a nomogram which gives the ratio S_(m)/e_(m) (in mm².μm⁻¹)for a palladium membrane as a function of the maximum allowable pressureP_(max) (in bar) and the desired lifetime FdV (in h);

FIG. 4 shows a nomogram which gives the ratio S_(m)/e_(m) (in mm².μm⁻¹)for stainless steel membrane as a function of the maximum allowablepressure P_(max) (in bar) and the desired lifetime FdV (in h);

FIG. 5 shows a nomogram which gives the ratio S_(m)/e_(m) (in mm².μm⁻¹)for an aluminum membrane as a function of the maximum allowable pressureP_(max)(in bar) and the desired lifetime FdV (in h).

FIG. 6 represents a schematic sectional view of a supercapacitor inaccordance with one embodiment;

FIG. 7 illustrates a lid similar to that of FIG. 6, with a differentembodiment of the membrane;

FIG. 8 illustrates a lid similar to that in FIG. 6 with a differentembodiment of the membrane; and

FIG. 9 illustrates an embodiment of a tubular membrane in accordancewith one embodiment.

DETAILED DESCRIPTION:

This object is achieved by using a membrane which is selectivelypermeable to H₂, with a high diffusion rate.

A supercapacitor according to the invention comprises a closed chamberwhich is equipped with means for exchanging a gas with the externalenvironment and in which two electrodes with a high specific surfacearea are placed while being separated by a separator, the separator andthe electrodes being impregnated with an electrolyte. The supercapacitoris characterized in that the means for exchanging a gas comprise amembrane which is permeable to hydrogen and its isotopes and impermeableto other gas species which are in the form of entities having a crosssection greater than or equal to 0.3 nm at the operating temperatures ofsupercapacitors, namely between −50° C. and 100° C.

A membrane used in a supercapacitor according to the invention has asurface area S (in m²) and a thickness (in m), and it consists of amaterial which is selected from among metals and metal alloys and theintrinsic permeability Φ of which (in mol.m⁻¹.s⁻¹Pa^(−1/2)) is selectivewith respect to hydrogen or its isotopes and has a value such that 10⁻¹⁵mol.s⁻¹.Pa^(−1/2)≦(Φ*S)/e≦10⁻⁹ g mol.s⁻¹.Pa^(−1/2), preferably 10⁻¹²mol.s⁻¹.Pa^(−1/2)≦(Φ*S)/e≦5. 10⁻¹⁰ mol.s⁻¹.Pa^(−1/2).

A material which satisfies the following Equation 1:10⁻¹⁵ mol.s⁻¹.Pa^(−1/2)≦(Φ*S)/e≦10⁻⁹ mol.s⁻¹.Pa^(−1/2)   Eq. 1defined above makes it possible to produce a membrane whose surface areais compatible with the dimensions of the supercapacitor.

In general, the permeability Φ (in mol.m⁻¹.s⁻¹.Pa^(−1/2)) depends on thenature of the gas/membrane pairing. Experimental measurements show thatΦ generally follows a law of the Arrhenius type

$\begin{matrix}{\Phi = {\Phi_{0}{\mathbb{e}}^{- \frac{E_{\Phi}}{T}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$in which Φ₀ is a constant (in mol.m⁻¹.s⁻¹.Pa^(−1/2)), E_(Φ) (in kelvin)is the activation energy of the diffusion, and T is the temperature (inK).

As indicated above, membranes are known which consist of a metallicmaterial capable of adsorbing hydrogen and diffusing it. Among thenumerous metallic materials, however, the majority cannot be used as anelement to prevent overpressure in a supercapacitor because they have atleast one of the following drawbacks: insufficient diffusion rate, lackof mechanical strength after adsorbing hydrogen, difficulty offeasibility with thicknesses ad hoc, loss of properties in the course oftime, cost. Intensive tests have been carried out by the inventors inorder to select, from among the materials capable of adsorbing anddiffusing hydrogen selectively, those which make it possible to use amembrane having a surface area compatible with the conventionaldimensions of supercapacitors, and in particular materials which satisfythe relation:10⁻¹⁵ mol.s⁻¹.Pa^(−1/2) <Φ*S/e<10⁻⁹ mol.s⁻¹.Pa^(−1/2)

The materials which fulfill the aforementioned criteria comprisemetallic materials in which the metals are selected from among Pd, Nb,V, Ta, Ni and Fe, and metal alloys of a metal selected from among Pd,Nb, V et Ta and at least one other metal selected from among Pd, Nb, V,Ta, Fe, Al, Cu, Ru, Re, Rh, Au, Pt, Ag, Cr, Co, Sn, Zr, Y, Ni, Ce, Ti,Ir, and Mo.

The membrane of a supercapacitor according to the invention may havevarious shapes, irrespective of the material from which it is made.

It may in particular be self-supported or non-self-supported. In theevent that it is self-supported, it preferably has a thickness greaterthan or equal to 5 μm.

In one embodiment, the membrane is a self supported membrane. Thisembodiment is particularly beneficial for materials which have a verylarge intrinsic selective permeability for hydrogen, and which canconsequently have a thickness sufficient to ensure mechanical strengthwhile guaranteeing compliance with Equation 1.

When the membrane consists of a material whose intrinsic permeabilityrequires the thickness to be reduced to a value which no longer providesthe membrane with sufficient mechanical strength, the membrane may belaid on a support layer or placed between two support layers. Thesupport layers consist of a material which has a very high permeabilityto hydrogen, this permeability not being selective. The multilayerstructure is such that the limits of the support layer or support layersdo not extend beyond the limits of the membrane. A multilayer structureis,represented in FIG. 2. The layer 2 constitutes the selectivemembrane. The layers 1 and 3 constitute the support layers. The surfacearea of the layer 2 must be greater than the surface layer of thesupport layer or each of the support layers, so that no gas can passthrough a support layer without also passing through the selectivemembrane. When the membrane is placed between two support layers, theselayers may consist of the same hydrogen-permeable material, oralternatively the material forming one of the support layers may bedifferent from the material forming the other layer. Thehydrogen-permeable material without selectivity may be selected fromamong polymers, ceramics, carbon and metals.

The selectively permeable membrane, as well as the support layers, mayconsist of a sintered material.

According to a particular embodiment of the invention, at least oneadditional layer is a sintered material having a thickness of more than0.3 mm (which makes it capable of withstanding a pressure of more than 2bar) and the membrane is a membrane made of palladium or apalladium-silver alloy having a thickness of from 0.03 μm to 10 μm and asurface area of from 0.0015 mm² to 10 mm², and the ratio S/e varies from0.05 mm²/μm to 1 mm²/μm.

The material of the additional, hydrogen-permeable layer or layers mayalso be a polymer or a mixture of polymers preferably having a thicknessof more than 0.005 mm, which makes it capable of withstanding a maximumpressure of 2.5 bar. In this case, the membrane is a membrane made ofpalladium or a palladium-silver alloy having a thickness of from 0.03 μmto 1 μm and a surface area of from 0.003 mm² to 1 mm², and the ratio S/evaries from 0.09 mm²/μm to 1 mm²/μm.

According to a particular embodiment, the hydrogen-permeable material ofthe additional layer or layers is a metal or a metal alloy, the membranehas a surface area of from 0.0007 mm² to 100 mm² and a thickness of from0.03 μm to 10 μm, and the ratio S/e varies from 0.025 mm²/μm to 0.1mm²/μm. The hydrogen-permeable material of the additional layer orlayers may in particular be palladium. In this case, the membrane has asurface area of from 0.0015 mm² to 1 mm², a thickness of 0.03 μm to 10μm and the ratio S/e varies from 0.05 mm²/μm to 0.1 mm²/μm.

A particularly preferred self supported membrane has a surface area ofbetween 0.15 mm² and 100 mm² and a thickness of from 5 μm to 100 μm, andthe ratio S/e varies from 0.03 mm²/μm to 1 mm²/μm.

In the case of a self supported membrane, that is to say one having athickness of more than 5 μm, an appropriate metallic material may beselected from among Pd, Nb, V and Ta. For any metal other than Pd,however, a continuous thin (thickness<1 μm) protective layer of Pdshould be applied on each of the faces of the membrane. Thus, accordingto a particular embodiment of the invention, the membrane consists of afilm of a metal selected from among Nb, V and Ta having a thicknessgreater than or equal to 5 μm, placed between two continuous palladiumfilms having a thickness of less than 1 μm. These palladium films may bedeposited by the conventional techniques of chemical, physical orelectrochemical deposition (CVD, PVD, electrochemical deposition) whichensure a continuous and regular deposit.

In a particular embodiment, the metallic membrane is self supported andconsists of palladium, it has a surface area of between 0.25 mm² and 10mm² and a thickness of greater than or equal to 5 μm, preferably from 5μm to 100 μm, and the ratio S/e varies from 0.05 mm²/μm to 0.1 mm²/μm. Amembrane having a thickness of 25 μm, a surface area of 1.5 mm² and aratio S/e of 0.06 mm²/μm is more particularly preferred.

In the case of a non-self supported membrane (thickness<5 μm), anappropriate metallic material may be selected from among Pd, Nb, V, Ta,Ni and Fe. For any metal other than Pd or Ni, however, a continuous thin(thickness<1 μm) protective layer of Pd should be applied on each of thefaces of the membrane. Thus, according to a particular embodiment of theinvention, the membrane consists of a film of a metal selected fromamong Nb, V, Ta and Ta having a thickness less than 5 μm, placed betweentwo continuous palladium films having a thickness of less than 1 μm.According to another embodiment, the membrane consists of a film ofpalladium or nickel having a thickness of less than 5 μm. As for theself supported membranes, the palladium may be deposited by theconventional deposition techniques.

Membranes may furthermore be mentioned which consist of an alloy of ametal selected from among Pd, Nb, V, Ta and at least one metal selectedfrom among Pd, Nb, V, Ta, Fe, Al, Cu, Ru, Re, Rh, Au, Pt, Ag, Cr, Co,Sn, Zr, Y, Ni, Ce, Ti, Ir and Mo. The alloys Pd₇₅Ag₂₅, Pd₉₂Y₈,Pd_(93.5)Ce_(6.5), Pd₆₀Cu₄₀, V₈₅Ni₁₅ stabilized with 0.05% yttrium ortitanium, V₅₃Ti₂₆Ni₂₁, V₅₀Nb₅₀, V ₁₃Cr₁₁Al₃Ti₇₃ (titanium alloy VC120),Nb₅₆Ti₂₃Ni₂₁, Nb₆₈Ti₁₇Ni₁₅ and Nb₉₉Zr₁ have particularly highpermeabilities. For example, the alloy V53Ti26Ni21 has a constant Φ₀ offrom 1.3 to 3.7.10⁻⁹ mol.m⁻¹.s⁻¹.Pa^(−1/2).

The chamber of a supercapacitor comprises a cylindrical side wall closedat each of its ends by a lid. The side wall is generally in the form ofa cylinder with a substantially circular base. The lids consist of aconductive material, generally a metallic material, and they areelectrically insulated from one another. If the side wall is anonconductive material, it serves as an insulator between the lids. Ifthe side wall consists of a metallic material, it is fixed to at leastone of the two lids by an insulating seal or an insulating adhesive. Itis also possible for one of the two lids and the side wall to form asingle part.

The membrane is fixed to the supercapacitor by various means selected asa function of the means for exchanging a gas, with which thesupercapacitor is fitted. Nevertheless, the membrane and its fasteningsystem in no way alter the hermeticity of the chamber of thesupercapacitor for substances other than hydrogen.

When the means for exchanging a gas are in the form of an opening in thechamber, and the membrane is a pellet of metal or a metal alloy, it maybe fixed on the chamber around said opening by welding, by brazing, bydiffusion brazing or by crimping.

A membrane in the form of a pellet may furthermore be forcibly insertedinto the means for exchanging a gas.

The membrane may be in the form of a metal tube closed at one of itsends, open at the other end and placed in the chamber so that its openend is fixed to the means for exchanging a gas, for example byinsulating seals or by adhesive bonding with the aid of an insulatingmaterial.

When the structure of the membrane comprises a polymer support layer, itmay be fixed on the gas exchange means by sealing, adhesive bonding orcrimping. However, the selectively permeable membrane must at leastcover the opening of the chamber.

The material constituting the membrane, as well as the dimensions of themembrane which are required for a given supercapacitor, may bedetermined by the person skilled in the art, in particular with the aidof the following data.

Internal pressure measurements on a supercapacitor without a leak haveshown that the internal pressure P (in Pa) increases proportionally tothe ageing time t according to Equation 3, in which k is a constant thatdepends on the temperature and the ageing voltage:P(t)=k t   Eq. 3Since hydrogen can be regarded as an ideal gas at the pressures inquestion, the following relation applies in which V₁ is the free volumeinside the supercapacitor (in m³), n_(H2 gas) is the quantity ofhydrogen gas (in mol) in the free volume V₁, R is the ideal gas constant(R=8.314 S.I.) and T is the temperature (in K):P(t)V ₁ =n _(H2 gas)(t)R T   Eq. 4A supercapacitor may furthermore contain a material which has hydrogenadsorption properties, for example activated carbons. The followingrelation is conventionally observed, in which m_(ads) is the mass (in g)of material which can adsorb hydrogen, n_(H2 ads) is the quantity ofhydrogen gas adsorbed (in mol) and ξ is a constant which depends on thetemperature:n _(H2 ads)(t)=ξm _(ads) P(t)   Eq. 5The quantity of hydrogen produced during ageing in a capacitor without aleak, denoted n_(H2), is therefore proportional to P, and therefore tothe time t, according to the following relation:

$\begin{matrix}{{n_{H\; 2}(t)} = {{{n_{H\; 2\mspace{14mu}{gas}}(t)} + {n_{H\; 2\mspace{14mu}{ads}}(t)}} = {{\left( {\frac{V_{1}}{R\; T} + {\xi\; m_{ads}}} \right){P(t)}} = {\alpha\; t}}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$The constant α depends on the temperature and the operating voltage ofthe supercapacitor, but it is not linked with the existence or absenceof a selective leak of hydrogen through the casing of thesupercapacitor.

In the case of a supercapacitor equipped with a membrane, a part ofn_(H2), denoted n_(H2 diff), diffuses through the membrane:n _(H2)(t)=n _(H2 gas)(t)+n _(H2 ads)(t)+n _(H2 diff)(t)   Eq. 7Fick's 1^(st) law gives the value of the flow rate

$- \frac{\mathbb{d}n_{H\; 2\mspace{14mu}{diff}}}{\mathbb{d}t}$through the membrane as a function of the surface area S_(m) and thethickness e_(m) of the membrane, as well as the intrinsic permeability Φof the material which constitutes the membrane, according to theequation:

$\begin{matrix}{\frac{\mathbb{d}n_{H_{2}\mspace{14mu}{diff}}}{\mathbb{d}t} = {\phi\frac{S_{m}}{e_{m}}\sqrt{P(t)}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$Differentiating Equation 7 gives a differential equation (Eq. 9) which,when solved, provides the variation, over time of the internal pressurein the supercapacitor equipped with a membrane (Eq. 8) (with the initialcondition P=0 at t=0):

$\begin{matrix}{\frac{\mathbb{d}n_{H\; 2}}{\mathbb{d}t} = {{{\left( \frac{V_{1}}{R\; T} \right)\frac{\mathbb{d}P}{\mathbb{d}t}} + {\xi\; m_{ads}\frac{\mathbb{d}P}{\mathbb{d}t}} + {\phi\frac{S_{m}}{e_{m}}\sqrt{P(t)}}} = \alpha}} & {{Eq}.\mspace{14mu} 9} \\{{{- 2}{\frac{\frac{V_{1}}{R\; T} + {\xi\; m_{ads}}}{\phi\frac{S_{m}}{e_{m}}}\left\lbrack {\sqrt{P(t)} + {\frac{\alpha\; e_{m}}{\phi\; S_{m}}{\ln\left( {1 - {\frac{\phi\; S_{m}}{\alpha\; e_{m}}\sqrt{P(t)}}} \right)}}} \right\rbrack}} = t} & {{Eq}.\mspace{14mu} 10}\end{matrix}$Equation 10 makes it possible to determine the limiting pressure valueinside the supercapacitor equipped with a membrane when the time tendsto infinity:

$\begin{matrix}{\left. \left. t\rightarrow\infty\Leftrightarrow\left. {1 - {\frac{\phi\; S_{m}}{\alpha\; e_{m}}\sqrt{P}}}\rightarrow 0 \right. \right.\Rightarrow P_{\lim} \right. = \left( \frac{\alpha\; e_{m}}{\phi\; S_{m}} \right)^{2}} & {{Eq}.\mspace{14mu} 11}\end{matrix}$Equation 10 also makes it possible to calculate the characteristics ofthe membrane as a function of the desired lifetime FdV of thesupercapacitor and the maximum allowable pressure P_(max), knowing thatP_(max)<P_(lim):

${{- 2}{\frac{\frac{V_{1}}{R\; T} + {\xi\; m_{ads}}}{\phi\frac{S_{m}}{e_{m}}}\left\lbrack {\sqrt{P_{\max}} + {\frac{\alpha\; e_{m}}{\phi\; S_{m}}{\ln\left( {1 - {\frac{\phi\; S_{m}}{\alpha\; e_{m}}\sqrt{P_{\max}}}} \right)}}} \right\rbrack}} = {FdV}$

The present invention is illustrated by the following exemplaryembodiments to which, however, it is not limited.

EXAMPLE 1

A membrane was calculated, for a supercapacitor having a capacitance of2600 F which has the following characteristics:

-   -   The chamber of the supercapacitor consists of a cylindrical        metal wall closed by two conductive lids, which form the poles        of the supercapacitor and are insulated from the metal wall by        an insulating material;    -   The chamber has a diameter of 7 cm and a height of 10 cm;    -   The free volume V₁ inside the supercapacitor is 50 cm³;    -   The electrolyte is a 1 M solution of tetraethylammonium        tetrafluoroborate (TEABF₄) in acetonitrile;    -   The separator is a cellulose film with a thickness of 25 μm        placed between two electrodes, each consisting of an aluminum        sheet carrying a layer of activated carbon, the separator being        in contact with the carbon layer of each electrode, and this        assembly being rolled up, the activated carbon having a total        mass of 100 g, this material furthermore constituting the mass        m_(ads) of hydrogen-adsorbing material inside the        supercapacitor, the adsorption efficiency ξ of which is 0.1        mmol.g⁻¹.bar⁻¹;    -   One of the electrodes is connected to one of the lids of the        chamber, and the other electrode is connected to the other lid        of the chamber;    -   The operating temperature T is 70° C.;    -   The ageing is determined under a constant voltage of 2.7 V,        which corresponds to a factor α of 0.15 mmol.h⁻¹;    -   The desired lifetime FdV is 2000 hours;    -   The maximum allowable pressure P_(max) inside the supercapacitor        is from 1 to 7 bar.        Palladium Membrane

If the material adopted for the membrane is palladium, Equation 2becomes:

$\Phi = {2.2\mspace{14mu} 10^{- 7}{\mathbb{e}}^{- \frac{1885}{T + 273}}{{mol} \cdot m^{- 1} \cdot s^{- 1} \cdot {Pa}^{- \frac{1}{2}}}}$

For a supercapacitor operating at 70° C., Φ is 9.10⁻¹⁰mol.m⁻¹.s⁻¹.Pa^(−1/2).

FIG. 3 shows a nomogram which gives the ratio S_(m)/e_(m) (inmm².μm^('1)) for a palladium membrane as a function of the maximumallowable pressure P_(max) (in bar) and the desired lifetime FdV (in h).

This FIG. 3 shows that a palladium membrane having a surface area of theorder of several mm² and a thickness of the order of one hundred μm(ratio S/e of the order of 0.05 mm²/μm) makes it possible to obtain thedesired result. A membrane of this type can be produced mechanicallywhen the supercapacitor contains a material that adsorbs hydrogen.

By way of comparison, the use of an aluminum membrane and a steelmembrane was considered.

Aluminum Membrane

If the material adopted for the membrane is aluminum, Equation 2becomes:

$\Phi = {3\mspace{14mu} 10^{- 5}{\mathbb{e}}^{- \frac{14800}{T + 273}}{{mol} \cdot m^{- 1} \cdot s^{- 1} \cdot {Pa}^{- \frac{1}{2}}}}$

For a supercapacitor operating at 70° C., Φ is 5.5.10⁻²⁴mol.m⁻¹.s⁻¹.Pa^(−1/2).

FIG. 5 shows a nomogram which gives the ratio S_(m)/e_(m) (in mm².μm⁻¹)for an aluminum membrane as a function of the maximum allowable pressureP_(max) (in bar) and the desired lifetime FdV (in h).

This figure shows that an aluminum membrane would require a surface areaof the order of several km² with a thickness of the order of one μm, toobtain the desired result. Such a membrane cannot therefore be envisagedin practice in a supercapacitor.

Steel Membrane

If the material adopted for the membrane is steel, Equation 2 becomes:

$\Phi = {1\mspace{14mu} 10^{- 7}{\mathbb{e}}^{- \frac{8000}{T + 273}}{{mol} \cdot m^{- 1} \cdot s^{- 1} \cdot {Pa}^{- \frac{1}{2}}}}$

For a supercapacitor operating at 70° C., Φ is 7.4.10⁻¹⁸mol.m⁻¹.s⁻¹.Pa^(−1/2).

FIG. 4 shows, a nomogram which gives the ratio S_(m)/e_(m) (in mm².μm⁻¹)for stainless steel membrane as a function of the maximum allowablepressure P_(max) (in bar) and the desired lifetime FdV (in h).

This figure shows that an aluminum membrane would require a surface areaof the order of several m² with a thickness of the order of one μm, toobtain the desired result. Such a membrane cannot therefore be envisagedin practice in a supercapacitor.

EXAMPLE 2

This example illustrates an embodiment of the invention in which aselectively permeable membrane is placed directly in contact with anorifice made in the lid of a supercapacitor.

FIG. 6 represents a schematic sectional view of a supercapacitor. Thesupercapacitor comprises a cylindrical side wall 13, a lid 4 comprisingan opening 10 on top of which there is a hollow stub 11, and a lid 14.The supercapacitor contains a coiled element 12, formed by winding anelectrode/separator/electrode multi-layer as described in Example 1. Theorifice 10 is intended to discharge the hydrogen which is formed duringoperation of the supercapacitor.

In the embodiment of FIG. 6, the selectively permeable membrane is afrit of a suitable material, for example palladium, pressed into theorifice 10.

EXAMPLE 3

This example illustrates a lid similar to that of FIG. 6, with adifferent embodiment of the membrane.

In FIG. 7, the lid 4 is represented during manufacture with its innerface on top. A palladium pellet 6 is placed on the central orifice 10, alayer 7 of a material to which the adhesive does not stick (for exampleEPDM) is placed over the pellet 6, and an adhesive 5 is applied in orderto hold the material 7 and the pellet 6 on the lid 4. The material 7 isthen removed, thus leaving the palladium free on both sides. The systemformed by the adhesive 5 and the palladium pellet 6 remaining afterremoval of the material 7 is a system which is selectively permeable tohydrogen.

A leak test was carried out on a supercapacitor fitted with such a lid,in order to test the leaktightness of the chamber of the supercapacitor.The test was carried out by the so-called “aspersion method” accordingto the following operating procedure.

Helium is injected into the chamber of the supercapacitor before closingit, then it is placed under the intake of a turbomolecular pumpintegrated with a leak tester of the ASM142 type. Under theseconditions, the helium molecules are small enough (molar mass 4 g.mol⁻¹)to be able to infiltrate rapidly into the micro-openings, cracks andpores of the casing of the element.

A measurement cell is mounted in series with the turbomolecular pump; itconsists of a magnetic deflection mass spectrometer specifically set upto detect the He²⁺ ions produced by ionization of the helium atoms inthe cell. The stream of helium ions detected in this way is convertedinto an overall leakage rate (through the cracks, pores andmicro-openings of the chamber of the supercapacitor). The leak isexpressed in mbar.l/s. It represents the quantity of helium whichescapes from the supercapacitor. The results are given in the tablebelow, for a control test (without a membrane to prevent overpressure)and the 5 tests with a membrane according to the invention, formed by apellet of palladium. The leakage level is so low that the chamber can beregarded as leaktight to helium and a fortiori other gases consisting ofentities larger than helium, and that hydrogen will be removed onlythrough the selective membrane.

Test No Control 1 2 3 4 5 Leak 1.5 10⁻⁹ 2.2 10⁻⁹ 1.9 10⁻⁹ 2.6 10⁻⁹ 1.910⁻⁹ 4.2 10⁻⁹ (mbar · l/s)

EXAMPLE 4

This example, which is represented in FIG. 8, illustrates a lid similarto that in FIG. 6 with a different embodiment of the membrane.

FIG. 8 represents a lid 4 of a supercapacitor (with the inner faceunderneath), a “seal 8 ₁/palladium pellet 6/seal 8 ₂” assembly placed ona central orifice of the lid, and a screw 9 which holds the assembly ontop of the orifice 10.

In order to confirm leaktightness of the system, a leak test was carriedout on 4 lids fitted with a device to prevent overpressure according toFIG. 7. The “leak” is expressed in mbar.l/s.

Test No Control 1 2 3 4 Leak 4.6 10⁻⁹ 3.4 10⁻⁹ 4.9 10⁻⁹ 3.00 10⁻⁹ 3.9010⁻⁹ (mbar · l/s)

A supercapacitor having a capacitance of 2600 F, comprising a device toprevent overpressure according to the present example, produced using apalladium pellet having a thickness of 100 μm and a diameter of 3 mm,exhibited a lifetime of 2000 hours. For comparison, a similarsupercapacitor without a device to prevent overpressure exhibited alifetime of 1000 hours, the two supercapacitors being used under thesame conditions.

EXAMPLE 5

This example illustrates an embodiment of a tubular membrane asrepresented in FIG. 9.

FIG. 9 represents a schematic sectional view of a supercapacitor. Thesupercapacitor comprises a cylindrical side wall 13, base 14 and a lid 4comprising an opening 10 on top of which there is a hollow stub 11; italso contains a coiled element 12 similar to that of Example 1. Theorifice 10 is intended to discharge the hydrogen which is formed duringoperation of the supercapacitor.

The membrane is formed by a tube 15 which is closed at one of its endsand open at the other end. The open end faces the opening 10 in the lid4. The tube 15 comprises a collar 16 around its open end, by which it isadhesively bonded against the inner wall of the lid around the opening10 with the aid of an insulating adhesive. The tube consists of a sheetof a suitable material, for example a sheet of palladium.

This embodiment substantially increases the surface area S of themembrane. It thus makes it possible to increase the thickness of themembrane in order to improve its mechanical strength, and/or to increasethe volume of hydrogen exchanged.

The invention claimed is:
 1. A supercapacitor comprising; a closedchamber which is equipped with means for exchanging a gas with theexternal environment; and in which two electrodes, each carrying a layerof activated carbon having a high specific surface area are placed whilebeing separated by a separator, the separator and the electrodes beingimpregnated with an electrolyte, wherein the means for exchanging a gashas a membrane which is permeable to hydrogen and its isotopes andimpermeable to gas species which have a cross section greater than orequal to 0.3 nm, at a temperature of between −50° C. and 100° C., and inthat the membrane has a surface area S (in m²) and a thickness e (in m),and in that it includes a material which is selected from the groupconsisting of metals and metal alloys, and the intrinsic permeability Φof which (in mol.m⁻¹.s⁻¹Pa^(−1/2)) is selective with respect to hydrogenand has a value such that 10⁻¹⁵ mol.s⁻¹.Pa^(−1/2)≦(Φ*S)/e≦10⁻⁹mol.s⁻¹.Pa^(−1/2).
 2. The supercapacitor as claimed in claim 1, wherein10⁻¹² mol.s⁻¹.Pa^(−1/2)≦(Φ*S)/e≦5 10⁻¹⁰ mol.s⁻¹.Pa^(−1/2).
 3. Thesupercapacitor as claimed in claim 1, wherein the metals are selectedfrom the group consisting of Pd, Nb, V, Ta, Ni and Fe and in that themetal alloys are selected from the group consisting of the alloys of ametal selected from among Pd, Nb, V and Ta and at least one other metalselected from the group consisting of Pd, Nb, V, Ta, Fe, Al, Cu, Ru, Re,Rh, Au, Pt, Ag, Cr, Co, Sn, Zr, Y, Ni, Ce, Ti, Ir and Mo.
 4. Thesupercapacitor as claimed in claim 1, wherein on at least one of itssurfaces the membrane carries an additional layer of ahydrogen-permeable material whose limits do not extend beyond the limitsof the membrane.
 5. The supercapacitor as claimed in claim 4, whereinthe membrane carries two additional layers of the samehydrogen-permeable material or two additional layers of differentmaterials.
 6. The supercapacitor as claimed in claim 4, thehydrogen-permeable material of the additional layer or layers isselected from the group consisting of polymers, ceramics, carbon andmetals.
 7. The supercapacitor as claimed in claim 4, wherein thehydrogen-permeable material of the additional layer or layers is a metalor a metal alloy, in that the membrane has a surface area of from 0.0007mm² to 100 mm² and a thickness of from 0.03 μm to 10 μm, and in that theratio S/e varies from 0.025 mm²/μm to 0.1 mm²/μm.
 8. The supercapacitoras claimed in claim 7, wherein the hydrogen-permeable material of theadditional layer or layers is palladium, in that the membrane has asurface area of from 0.0015 mm² to 1 mm², and in that the ratio S/evaries from 0.05 mm²/μm to 0.1 mm²/μm.
 9. The supercapacitor as claimedin claim 4, wherein at least one of the elements among the membrane andthe additional layer or layers is a sintered material.
 10. Thesupercapacitor as claimed in claim 9, wherein at least one additionallayer is a sintered material having a thickness of more than 0.3 mm, andin that the membrane is a membrane made of palladium or apalladium-silver alloy having a thickness of from 0.03 μm to 10 μm and asurface area of from 0.0015 mm² to 10 mm², and in which the ratio S/evaries from 0.05 mm²/μm to 1 mm²/μm.
 11. The supercapacitor as claimedin claim 6, wherein the hydrogen-permeable material of the additionallayer or layers is a polymer or a mixture of polymers having a thicknessof more than 0.05 mm, and in that the membrane is a membrane made ofpalladium or a palladium-silver alloy having a thickness of from 0.03 μmto 1 μm and a surface area of from 0.003 mm² to 1 mm², and in which theratio S/e varies from 0.09 mm²/μm to 1 mm²/μm.
 12. The supercapacitor asclaimed in claim 1, wherein the membrane is a self supported membrane.13. The supercapacitor as claimed in claim 12, wherein the membrane hasa thickness greater than or equal to 5 μm.
 14. The supercapacitor asclaimed in claim 12, wherein the membrane has a surface area of between0.15 mm² and 100 mm² and a thickness of from 5 μm to 100 μm, and in thatthe ratio S/e varies from 0.03 mm²/μm to 1 mm²/μm.
 15. Thesupercapacitor as claimed in claim 13, wherein the membrane is apalladium membrane, in that it has a surface area of between 0.25 mm²and 10 mm², and in that the ratio S/e varies from 0.05 mm²/μm to 0.1mm²/μm.
 16. The supercapacitor as claimed in claim 15, wherein saidpalladium membrane has a thickness of 25 μm and a surface area of 1.5mm², and in that the ratio S/e is 0.06 mm²/μm.
 17. The supercapacitor asclaimed in claim 12, wherein the membrane consists of a film of a metalselected from the group consisting of Nb, V and Ta having a thicknessgreater than or equal to 5 μm, placed between two continuous palladiumfilms having a thickness of less than 1 μm.
 18. The supercapacitor asclaimed in claim 1, wherein the membrane is a non-self supportedmembrane consisting of a palladium or nickel film having a thickness ofless than 5 μm.
 19. The supercapacitor as claimed in claim 1, whereinthe membrane is a non-self supported membrane consisting of a film of ametal selected from among Nb, V, Ta, and Fe, having a thickness of lessthan 5 μm, placed between two continuous palladium films having athickness of less than 1 μm.
 20. The supercapacitor as claimed in claim3, wherein the alloy is selected from the group consisting of Pd₇₅Ag₂₅,Pd₉₂Y₈, Pd_(93.5)Ce_(6.5), Pd₆₀Cu₄₀, V₈₅Ni₁₅ stabilized with 0.05%yttrium or titanium, V₅₃Ti₂₆Ni₂₁, V₅₀Nb₅₀, V₁₃Cr₁₁Al₃Ti₇₃, Nb₅₆Ti₂₃Ni₂₁,Nb₆₈Ti₁₇Ni₁₅ and Nb₉₉Zr₁.
 21. The supercapacitor as claimed in claim 1,wherein the membrane is a metal tube which is closed at one of its ends,open at the other end and placed in the chamber so that its open end isfixed to the means for exchanging a gas.
 22. The supercapacitor asclaimed in claim 1, wherein the means for exchanging a gas are in theform of an opening in the chamber.
 23. The supercapacitor as claimed inclaim 22, wherein the membrane consists of a pellet of metal or a metalalloy, which is fixed around said opening by welding, brazing, diffusionbrazing or crimping.